control system

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RESEARCH ARTICLE

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DOI http://dx.doi.org/10.4038/jnsfsr.v42i3.7395

5726 EDVHG HPEHGGHG FRQWUROOHU LPSOHPHQWDWLRQ RI D ELODWHUDO

control system

* Corresponding author ([email protected])

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Department of Electrical Engineering, Faculty of Engineering, University of Moratuwa, Katubedda, Moratuwa.

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law of action and reaction between the human and the remote environment. In this research, an embedded system based bilateral control was developed on a microcontroller based PRWRUGULYHUSODWIRUP%LODWHUDOWHOHRSHUDWLRQQHHGVUHDOWLPH

representation of the forces and positions. This challenge was DFFRPSOLVKHG E\ LQFRUSRUDWLQJ D UHDOWLPH RSHUDWLQJ V\VWHP

(RTOS) with a resource limited microcontroller. Sensorless WRUTXHGHWHFWLRQWHFKQLTXHZDVXVHGWR¿QGWKHH[WHUQDOWRUTXH

while disturbance observer was used as a disturbance rejection tool and also to improve the system’s robustness. When the motion controller was implemented on a microcontroller,

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an ideal bilateral control, force/position errors should be zero.

The experimental results in contact motion showed a force error OHVVWKDQDQGDSRVLWLRQHUURUOHVVWKDQZKLFKYDOLGDWHV

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controllers have never been successfully implemented using a microcontroller.

.H\ZRUGV Bilateral control, disturbance observer, haptics, microcontroller, reaction torque observer, real time operating system.

INTRODUCTION

Well before the dawn of the modern age, information was recorded in a pictorial language consistent with WKHVLPSOHOLIHVW\OH7KHQH[WVLJQL¿FDQWGHYHORSPHQW

was the storage methods for sound information. This was followed by the development of visual recording technology, especially after the invention of the television.

In the modern world, communication technologies have grown rapidly with the boom in the Internet enabled transmission of information including text, sound and YLGHR¿OHVZRUOGZLGH$OWKRXJKDXGLRDQGYLVXDOIRUPV

perceived by humans can be stored and reproduced in

a remote place, the senses of nose (smell) and tongue (taste) cannot normally be transmitted nor stored in electronic means.

This paper focuses on the transmission of sense of touch (haptic) information by electronic means from a remote point. This technology has been developed VSHFLDOO\E\PDQ\UHVHDUFKHUVRIWKH2KQLVKL/DERUDWRU\

of the Keio University, Japan (Nishimura et al., 2006;

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would be a TV remote controller where the human presses a channel number and the control signal travels unilaterally from the TV controller to the TV set, where the channel is changed. Four senses of the human, namely smell, vision, hearing and taste can be considered as unilateral and the sensing is done unilaterally as shown in Figure 1. However, the sense of touch is not unilateral.

It obeys the law of action and reaction and the concept is shown in Figure 2.

Haptic sensation is a realization of the law of action and reaction (Katsura et al., 2007) between the human and the environment. If the sense of touch has to be transmitted to a remote place, then the reaction force from the environment should be felt by the operator in UHDOWLPH 7KHUHIRUH WKHUH VKRXOG EH D VSHHG\ ZD\ RI

communication between the two points once the object and the operator are kept away. There should be two identical manipulators at both ends: to sense the environment’s reaction and to transmit it back to the operator; and to transmit the operator’s intention to the environment

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September 2014 Journal of the National Science Foundation of Sri Lanka 42(3)

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have proved that it could be effectively used as a force/

torque sensor in DC motor based applications. In the above research they have used computer based systems ZLWK KLJKHQG SURFHVVLQJ FDSDELOLWLHV DV WKH ELODWHUDO

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and reaction. In this paper, the authors propose a novel embedded system based implementation of a bilateral WHOHRSHUDWLRQV\VWHP0LFURFRQWUROOHUEDVHG5726KDV

been used to accomplish the challenging time constraints DFWLRQDQGUHDFWLRQ7KLVZDVDVLJQL¿FDQWDFKLHYHPHQW

as the bilateral controllers have never been successfully implemented using a microcontroller.

Conventional bilateral control systems use force sensors. In this research, a reaction torque observer 572% ZDV XVHG DV WKH IRUFH VHQVRU$ PRWRU GULYHU

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EDVHGFXUUHQWFRQWUROOHUGHYHORSHGHDUOLHU$EH\NRRQ Senevirathne, 2012) has been used in this study.

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Disturbance observer is a tool for compensating disturbances, which is very useful and widely utilized in motion control applications of robotics. When applying a force to an object via a motor, we would expect the object to accelerate according to the Newton’s second law as shown in the equation (1)

J τ

ref

θ =

+

=

...(1)

where,

θ =

=

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J τ

ref

=

+

=

UHIHUHQFHPRWRUWRUTXH

J

PRWRULQHUWLD

)LJXUH Transmission of real world haptic information

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real world haptic information communication between the operator and the real environment is in two directions, or bilateral.

If there is to be a successful solution for the transmission of haptic information then it should be bilateral. The bilateral control is a realization of the law of action and reaction between the master system, which the operator manipulates and the slave system, which is in contact with the environment (Katsura, 2004; Shimono,

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In control systems unknown signals affect the SHUIRUPDQFH XQPRGHOOHG RU XQGH¿QHG VLJQDOV ZKLFK

are generated may contribute to the disturbance. Usually friction and parameter changes of the process over time, may create another type of disturbance in the system.

But to increase the system robustness, to attain high accuracy, and to achieve fast response, compensation of WKHGLVWXUEDQFHLVYHU\LPSRUWDQW$VH[SODLQHGDERYH

bilateral control works based on the law of action and reaction. To realize bilateral control, force/position information should be exchanged between the remote points very fast (Katsura et al., 2007). Such systems are currently realized using very fast computer based processors having many resources (Katsura, 2004;

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proposed by Ohnishi et al. (1996), Katsura (2004) and many others. Some research groups at the Keio University (Ohnishi et al., 1994) have used disturbance observer (DOB) to enhance the system robustness.

)LJXUH Transmission of sense of touch

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Embedded RTOS - based bilateral control system 219

Journal of the National Science Foundation of Sri Lanka 42(3) September 2014

When the motor is experiencing an external disturbance, the motor system can be represented by equation (2)

J

J J

dis

ref τ

θ=τ +

∆ +

= =

∆

...(2)

Where,

J

J J

τdis

+

∆ +

=

∆

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and estimates the disturbance, which is not normally measurable otherwise. Once the unknown disturbance is measured, it compensates for the disturbance. Its net effect on the system is a near “zero” disturbance through WKH HIIHFW RI WKH FRPSHQVDWLRQ $EH\NRRQ 2KQLVKL

2006).

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in the system without using any force sensors (Ohnishi et al., 1994;1996).

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follows,

J

J J

J θ = T

_m

− T

_l

∆ +

= = +∆

−

=

∆

...(3)

where;

Tm± PRWRU WRUTXHT_l± ORDG WRUTXHJ± PRWRU LQHUWLD

θ=

=

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When a servo motor with one degree of freedom is considered, it could be represented as shown in Figure 3.

and viscosity terms; friction terms can be estimated as in Ohnishi et al. (1994).

Therefore for a DC servo motor, equation (3) could be rewritten as,

)

( _int

θ

K I T T T B

J = _t _f − + _ext+ _f +

∆ +

= =

=

∆

...(5)

Equation (5) has two parameters, namely J and torque constant

K

_t. Inertia can be changed due to the PHFKDQLFDOFRQ¿JXUDWLRQRIWKHV\VWHPDQGWKHQRPLQDO

inertia, which is available can be different from the manufacturer’s given value.

J J

J =

_n

+ ∆

...(6)

Similarly parameter

K

_t also changes,

∆ +

= K

t

= K

tn

+ ∆ K

t

−

=

∆

...(7)

Here

J

_n and

∆ +

= = K +

tn

=

∆

are nominal inertia and the nominal torque constant of the motor, respectively.

Disturbance torque

T

_dis is represented as

ref t l

dis T J K I

T = +∆

θ

−∆

...(8) By applying the value of

T

_l in the above equation (8),

ref t f

ext

dis

T T T B J K I

T = + + + θ + ∆ θ − ∆

int

−

=

∆

...(9) where,

) ( J J

_n

J = −

∆

)

( _tn _t

t K K

K = −

∆

)LJXUH Motor with disturbance torque

The load torque is considered as,

θ B T T T

T

_l

= +

_ext

+

_f

+

int

∆ +

=

∆

...(4)

T

int WKH LQWHUQDO WRUTXH GHULYHG IURP WKH /DJUDQJH

motion equation consists of the inertia torque and the gravity effect.

T

_ext consists of the torque external to the system. Friction term

θ B T_f +

+

∆ +

=

∆

is the sum of Coulomb

)LJXUH Block diagram of disturbance observer

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disturbance feedback it can compensate for the unknown disturbance acting on the system (Shimono et al., 2005;

Senevirathne et al., 2012). If the frictional components are measured and eliminated from the disturbance output, the real reaction torque

T

_reccan be measured. This is a variant of the disturbance observer, and it is called the reaction torque observer (RTOB).

Functional block diagram of the reaction torque observer is shown in Figure 5. This block diagram is similar to the disturbance observer except for the friction components.

The disturbance torque, which is estimated by the disturbance observer is depicted in equation (9) from which the friction components

f B

T + +

+ θ

∆

are subtracted (Ohnishi et alDQG¿QDOO\HTXDWLRQFDQEH

derived.

)) (

( _ref _tn _dis _ext

rec rec

rec I K J T T

g s

T g −∆ − −

+

=

θ

...(11)

Here Tˆ_rec is the estimated reaction torque and grec is the observer gain of reaction torque observer.

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the master and the slave sides from the responses of the slave and master sides, respectively (Sumiyoshi &

Ohnishi, 2004). These responses are converted as inputs to the master and slave sides. The slave side is controlled using the torque/position feedback of the master side and vice versa$EH\NRRQ 2KQLVKLDVVKRZQ

in Figure 6.

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high transparency. In simple terms, following the literal meaning of the word “transparency”, it means how well the torque/position information of the slave side is transparent to the master side (Yokokohji & Yoshikawa, 1994). In other words, transparency is the match of the impedance perceived by the operator with the HQYLURQPHQW/DZUHQFH

The block diagram of disturbance observer is shown in Figure 4.

Then the disturbance torque is estimated by the disturbance REVHUYHUWKURXJKWKHORZSDVV¿OWHUDVHTXDWLRQ

Tˆ

_dis dis

dis dis

dis

T

g s

g )

(

...(10)

Here

Tˆ

_dis is the estimated disturbance torque and

g

_dis

is the observer gain. The disturbance observer calculates and estimates the reaction torque as quickly as possible (Ohishi et al., 1987; Sabanovic, 2003). By having the

)LJXUH Block diagram of RTOB

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bilateral controllers cannot achieve transparency and stability simultaneously due to the uncertainties of the system and the environment (Katsura et al., 2008).

Therefore, a bilateral system should strike a balance EHWZHHQ WKH VWDELOLW\ DQG WKH WUDQVSDUHQF\ +DVKWUXGL

zaad & Salcudean, 2002a;b). In conventional bilateral control, force sensors are used to detect the force of master and slave. For force control, many researchers XVH IRUFH VHQVRUV WR GHWHFW H[WHUQDO IRUFHV *HRUJH

et al., 1983; Takeo & Kosuge, 1997). Strain gauges are widely used as force sensors (Mayer et al., 2005) since strain demonstrates the external force. However, when it experiences motion with contact, usually the accuracy of WKHRXWSXWGHWHULRUDWHV$GGLWLRQDOO\IRUFHVHQVRUVKDYH

narrow bandwidths. Therefore, the force sensation sensed through the force sensors is often dull (Katsura, 2004).

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sensor is located. In bilateral control applications, it LVYHU\GLI¿FXOWWR¿[WKHIRUFHVHQVRUZKHUHWKHIRUFH

should be sensed. Therefore, force sensors are often criticized, because they cannot produce a vivid sensation of the force (Katsura, 2004). Force sensor itself draws power from the system’s measurand, which distorts the force to be measured. On the other hand, the disturbance observer is capable of providing a much wider bandwidth than the force sensors, because it is possible to set the sampling time short and set the observer gain high.

Therefore, disturbance observer is more suitable for force measurement than force sensors. The bandwidth of the RTOB based force control was analyzed by Katsura et al., (2006). They have experimented with the frequency EDQGZLGWK RI VHQVHG IRUFH +] VXFFHVVIXOO\

However, in this research the authors have used a lower EDQGZLGWK +] DV WKH KXPDQ LQWHUDFWLRQ ZLWK D

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bandwidth could not be obtained due to the resource constraints of the microcontroller.

Bilateral control is a realization of natural law of motion of two objects. But the input from the operator or the environmental response consists of elements of disturbance and not only the force. Therefore, to realize a good bilateral system, a good disturbance rejection mechanism should be in action. Robust motion control with bilateral control is achieved through the disturbance REVHUYHU$FFHOHUDWLRQ FRQWURO SOD\V DQ LPSRUWDQW UROH

in realizing this. Bilateral control structure is derived based on the law of action and reaction (Katsura et al., 2008). When the operator manipulates the system from the master side, the slave side has to be in contact with the environment. However, in an application where the master and slave are different, scaling may be used.

)LJXUH Basics of bilateral control Operator Environment

Figure 6 shows the basics of bilateral control. For bilateral control without scaling, the following equations could be derived.

0 =

−

s

m

θ

...(12)

=

− θ

θ + = 0

s

m

τ

...(13)

Here subscript m denotes the master and subscript s denotes the slave. There are many scaling methods such as position, torque, impedance and time scaling (Itoh et al., 2000; Tsuji & Ohnishi, 2004). In bilateral control it is necessary to attain both the ideal force control and the position control simultaneously. The equations (12) and (13) are transformed to (14) and (15) as accelerations.

0 =

−

s

m

θ

...(14)

0 = +

_s

m

θ

...(15)

Since disturbance observer is used as a force sensor in bilateral control, the sampling time should be small enough for a better response. The relationship of torque, which is represented as the acceleration is called the common mode between the master and the slave system, i.e. equation (15). Equation (14), which represents the position relationship is called as differential mode. This can be attributed to the sign of equations. Therefore, it is possible to realize both force control and position control independently in bilateral control (Shimono et al., 2005), when force and position are combined in the acceleration dimension. Equation (14) and (15) can be transformed into the following equations based on the optimal goal of bilateral control.

0 →

=

−

s com

m

θ θ

θ

...(16)

0 →

=

+

s dif

m

θ θ

θ

_...(17)

Disturbance observer and reaction torque observer are used in both master and slave sides as well. Equations (12) to (17) denote the basics of the bilateral control.

DOB, RTOB and the basics of bilateral control could be represented as shown in Figure 7.

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)LJXUH Hardware experimental systems )LJXUH Block diagram of conventional bilateral control

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&216758&7,21 Hardware

The bilateral control functionality was experimented on a hardware setup consisting of master and slave manipulator units. Two similar DC motors (by Electrocraft Inc.) were used for each unit. Figure 8 shows the arrangement of the hardware experimental platform.

The hardware platform consists of two PWM driven motor drivers with two driver ICs (DRV8432 by Texas ,QVWUXPHQWZKLFKFDQFDUU\DFXUUHQWXSWR$ZLWK

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microcontroller. Position sensing is done by two encoders coupled with master and slave manipulators, having 5000 ppr and 2500 ppr, respectively. The hardware functional block diagram of the above system is shown in Figure 9.

)LJXUH Hardware functional block with current sensor module

Current sensor module operation

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Inc.). Figure 9 shows the functionality of a current sensor, ZKLFKDOVRVHQVHVDFXUUHQWXSWR$LQERWKGLUHFWLRQVDV

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direction through the current sensor; The blue colour

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direction taken as the positive direction while the red colour direction taken as negative direction. When current ÀRZVLQWKHSRVLWLYHGLUHFWLRQWKHRXWSXWLQFUHDVHVIURP

“zero current voltage” set at 2.5 VDC, with a gradient of

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positive direction. The voltage difference of 0.8 V is fed WRDGLIIHUHQWLDODPSOL¿HUZKLFKKDVDQDPSOL¿HGRXWSXW

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the current sensor is also the same as that of the positive direction.

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zero current is ~2.5 V. The output of the current sensor varies positively or negatively around 2.5 V (from 1.7

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This output is fed into high impedance input, differential DPSOL¿HU WR DPSOLI\ ZLWK UHVSHFW WR WKH GLUHFWLRQ RI

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and current consumption at the sensor output.

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Normally, the “zero current voltage” (~2.5 VDC) is applied to the reference terminal, while the output from the current sensor is fed into the input terminal of the GLIIHUHQWLDO DPSOL¿HU 7KH QHW LQSXW WR WKH GLIIHUHQWLDO

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feeding into the microcontroller. The reference supply is always maintained constantly throughout the operation.

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research has limited resources. In order to optimize the resources of the microcontroller, a real time operating system (RTOS) is incorporated. The motor torques are determined by measuring the current through the motors.

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two motors give four current output signals, which are fed into four analog input pins of the microcontroller.

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The controller implemented in the microcontroller was HYDOXDWHG E\ DQ H[SHULPHQW ZLWK DFWXDO PDVWHUVODYH

manipulators. In this experiment, the master manipulator is operated by a human, where the slave is in an unknown

Parameter Value Unit

Rated output 0.2 kW

Rated/maximum torque 20.5 /169.5 N cm Encoder resolution

Master 5000 Pulses/rev

Slave 2000 Pulses/rev

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Parameters Symbol Value Units

Motor inertia J_n 0.268 kgcm²

7RUTXHFRHI¿FLHQW K_t 1FP$PS

Proportional constant K_p 0.01 rad/sec

Integral constant K_i 0.0 rad/sec

Derivative constant K_d 0.001 rad/sec

&XWRIIIUHTXHQF\ORZ K_dis +]

SDVV¿OWHU

7DEOH Experimental parameters

environment. Master and slave manipulators are controlled by the bilateral controller.

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real time operating system (RTOS) with a sampling WLPHRIȝV7LPHFULWLFDOPDLQFRQWUROSURJUDPPH

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motors are given in Table 1 and the parameters used in the experiment are shown in Table 2.

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contact motions of the bilateral control system. In this setup, the system was operated with a force bandwidth of

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were conducted with two different observer gains: g_dis = 50 rad/s and g_dis = 200 rad/s.

In this experimental setup, there are two main units:

master and slave manipulators. The master side was operated by human interaction. In an ideal bilateral WHOHRSHUDWLRQWKHSRVLWLRQDQGWKHWRUTXHDSSOLHGDWWKH

master manipulator should appear as it is in the slave manipulator. In this test, slave side manipulator was not in contact with any material. Thus it was freely movable.

In the contact motion test, the slave unit is placed in an environment where the manipulator movement comes LQFRQWDFWZLWKDQREMHFWQ\ORQIRDP)LJXUHV±

show the experimental results of position and torque UHVSRQVHVLQERWKFRQWDFWDQGQRQFRQWDFWPRWLRQ

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)LJXUH 3RVLWLRQHUURUUHVSRQVH*GLV UDGV )LJXUH 7RUTXHHUURUUHVSRQVH*GLV UDGV )LJXUH Torque response, g_dis = 200 rad/s

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Figures 10 and 12, clearly shows that the position response of the slave manipulator is closely following the position response of the master manipulator. In both experiments (for different observer gains; g_dis= 50 rad/s,

g_dis UDGVSRVLWLRQHUURUVDUHLQVLJQL¿FDQWO\VPDOOHU

as shown in Figure 15 (maximum position error is less WKDQ

The torque responses are shown in Figures 11 and 13.

The master and slave torques follow the same pattern;

the mirror image of the other (as per equation 13). The response of the slave manipulator closely follows the torque applied at the master manipulator as shown in Figure 11. Figures 11 and 13 show the torque responses LQGLIIHUHQWFXWRIIIUHTXHQFLHV'2%JDLQ$KLJKHUFXW

off frequency (Figure 13) shows high frequency variations of the torque when compared to the Figure 11.

Contact motion test

Torque response of the slave manipulator in contact motion has followed the master manipulator as shown in Figures 11 and 13. Figure 14 shows the error bar JHQHUDWHG RQ WRS RI WKH ³PDVWHU´ WRUTXH FXUYH$V IRU

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contact motion test.

Position response of the slave manipulator in contact motion has followed the master manipulator as shown in Figures 10 and 12. The result of the contact motion test shows a considerable error in position response.

This was due to the low proportional gain selected in the controller. However, it could not be increased as for WKHWRUTXHOLPLWVRIWKHPRWRU$FFRUGLQJWRWKHHUURUEDU

shown in Figure 15, the position response error is always OHVVWKDQ

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In this research, the bilateral control system was realized by using a microcontroller in which the available resources in the processor were limited compared to other existing high speed processors.

The operation bandwidth of the torque response was

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follows the master manipulator’s position. Torque response in contact motion shows a lower error compared

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incorporating high speed processors and maintaining a smaller sampling time. Sampling time was kept constant E\XVLQJWKHPLFURFRQWUROOHUEDVHG5726$PRWRUGULYHU

system was developed with a current controller to meet the embedded system’s requirements. This system can be specially used in embedded systems based bilateral controllers where the mobility is required.

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The authors would like to express their gratitude to MBED (mbed.org) for providing microcontrollers for this research.

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for bilateral motion control. PhD thesis, Keio University, Japan.

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of environmental information. PhD thesis, Keio University, Japan.

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observer. IEEE Transactions on Industrial Electronics 55í

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13. Katsura S., Suzuyama T. & Ohnishi K. (2007). Bilateral WHOHRSHUDWLRQZLWKGLIIHUHQWFRQ¿JXUDWLRQXVLQJLQWHUDFWLRQ

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